Recently, it was reported that surficial nanostructures of black silicon, wings of cicadas and dragonflies have a bactericidal activity (Non-patent Document 1). Reportedly, the physical structure of the nanopillars that black silicon and wings of cicadas and dragonflies have produces a bactericidal activity.
According to Non-patent Document 1, black silicon has the strongest bactericidal activity on Gram-negative bacteria, while wings of dragonflies have a weaker bactericidal activity, and wings of cicadas have a still weaker bactericidal activity. Black silicon has 500 nm tall nanopillars. Wings of cicadas and dragonflies have 240 nm tall nanopillars. The static contact angle (hereinafter, sometimes simply referred to as “contact angle”) of the black silicon surface with respect to water is 80°, while the contact angles of the surface of wings of dragonflies and cicadas with respect to water are 153° and 159°, respectively. It is estimated that black silicon is mainly made of silicon, and wings of dragonflies and cicadas are made of chitin. According to Non-patent Document 1, the composition of the surface of black silicon is generally a silicon oxide, and the composition of the surface of wings of dragonflies and cicadas is generally a lipid.